In some cases, it is necessary to estimate the state quantity of a skid motion of a vehicle such as the skid angle or skid speed of the center of gravity of the vehicle for behavior control or the like of the vehicle. Then, as the estimation technique thereof, there has been known a technique disclosed in, for example, Japanese Patent No. 3669668 (hereinafter, referred to as Patent Document 1). According to this technique, a road surface reaction force (friction force) acting on each wheel of the vehicle from a road surface is estimated by using the estimated value of the friction coefficient and a friction characteristic model of the wheel while estimating the friction coefficient of the road surface. Then, the lateral acceleration of the center of gravity of the vehicle, which is dynamically generated, is estimated on the basis of the total sum of forces in the lateral direction of the vehicle (resultant force) of the estimated road surface reaction force. Further, the temporal change rate of the skid angle of the center of gravity of the vehicle is estimated on the basis of the dynamic motion model of the vehicle from the estimated value of the lateral acceleration, the detected value of the vehicle speed, and the detected value of the yaw rate of the vehicle. The estimated value of the skid angle of the center of gravity of the vehicle is found by integrating the temporal change rate.
In the technique of estimating the road surface reaction force (friction force) acting on each wheel of the vehicle and estimating the state quantity of the skid motion of the vehicle by computing a dynamic motion model on the basis of the estimated value of the road surface reaction force as disclosed in Patent Document 1, the estimated value of the road surface reaction force affects the estimated value of the state quantity of the skid motion of the vehicle as a result.
Further, the estimated value of the road surface reaction force is susceptible to the effect of the nonlinearity of the friction characteristics of wheels (the friction characteristics to the road surface) and an error of the friction characteristic model or the like. Particularly, during the transitional period in which the friction coefficient of the road surface or the motion state of the vehicle changes, generally the accuracy of the estimated value of the road surface reaction force easily decreases. Consequently, the accuracy of the estimated value of the state quantity of the skid motion of the vehicle easily decreases. In addition, the road surface reaction force acting on each wheel of the vehicle is affected by the bank angle (cant angle) of the road surface, and even if the state quantity of the skid motion of the vehicle is constant, the road surface reaction force changes according to a change in the bank angle (cant angle) of the road surface.
The technique disclosed in Patent Document 1, however, does not include a technique for compensating for the effect of the decrease in the accuracy of the estimated value of the road surface reaction force during the transitional period in which the friction coefficient of the road surface or the motion state of the vehicle changes and the effect of the change in the road surface reaction force caused along with the change in the bank angle of the road surface. Accordingly, the technique disclosed in Patent Document 1 has a disadvantage that the accuracy of the estimated value of the state quantity of the skid motion of the vehicle easily decreases particularly during the transitional period in which the friction coefficient of the road surface or the motion state of the vehicle changes or in which the bank angle of the road surface changes.